This invention relates generally to pumps, and more particularly concerns a new piston pump.
Pumps raise the pressure of fluids that are mostly in the liquid phase. When reference is made to a fluid in the following description, what is meant is a liquid with, possibly, some vapor content. When reference is made to a liquid, what is meant is a fluid that is purely in the liquid phase. Two types of pumps have been used: (1) positive displacement pumps and (2) centrifugal pumps. A positive displacement pump pushes on a piston that pressurizes and displaces the fluid in a pumping chamber. A centrifugal pump raises the pressure of the fluid by accelerating the fluid radially outward with an impeller to a surrounding diffuser, where the fluid""s high velocity is converted to a high pressure. Both of these types of pumps have problems associated with their use.
Positive displacement pumps have problems when they are used to pump cryogenic fluids (fluids at temperatures less than about 200xc2x0 K., or xe2x88x92100xc2x0 F.). One such problem occurs because a piston rod extends from the piston to a drive mechanism, such as a motor. The piston is in contact with the cryogenic fluid (cold) and the motor is at the ambient temperature or higher (warm). The difference in temperature causes heat conduction to the cryogenic fluid. It is typically expensive to cool cryogenic fluids, so adding heat to the cryogenic fluid is undesirable. Also, the addition of heat can boil the liquid content of the cryogenic fluid, which can reduce the performance of the pump. In addition, positive displacement pumps for cryogenic applications have used lubricated piston sleeves. The piston sleeves rub against the cylinder wall to create a rubbing seal. The piston sleeves are made from a different material than the cylinder wall. When the materials are cooled, the materials from which the piston sleeves are made contract to a greater extent than the materials from which the cylinder walls are made. Therefore, even if there is no gap between the sleeves and the cylinder walls at room temperature, there are significant gaps after the sleeves and the cylinder walls have cooled to their operating temperature. The large gaps lead to intolerably high leakage. For example, if a stainless steel cylinder has an inside diameter of 2.000 inches at 77xc2x0 F., then it has an inside diameter of about 1.997 inches at 112xc2x0 K. If the piston sleeve has an outside diameter of 2.000 inches at 77xc2x0 F. (zero clearance between it and the cylinder wall), then it could have an outside diameter of about 1.978 inches at 112xc2x0 K. The gap of 1.997xe2x88x921.978=0.019 inches at 112 K would result in intolerably high leakage.
Centrifugal pumps have several problems. The major problem is cavitation. Cavitation is the formation of gas or vapor-filled bubbles resulting from a drop in pressure of a fluid without a change in temperature. Cavitation reduces the pressure rise of centrifugal pumps. Also, when cavitation occurs, the tiny bubbles erode the impellers. In addition, cavitation can create imbalance in the impeller and cause bearing failure. Cavitation is most likely to occur (and, therefore, especially a problem) when the fluid must be pumped from a temperature close to its saturation temperature. For example, in steam boiler systems, the feedpump must pump feedwater from the deaerator (at about 20 psia) to the boiler pressure (often above 1,000 psia). The water in the deaerator portion of the system is saturated, so the centrifugal feedpumps can have cavitation problems. Cavitation is also a problem when centrifugal pumps are used to pump cryogenic fluids, because cryogenic fluids often must be pumped from storage at temperatures at or near saturation. Also, cryogenic centrifugal pumps often have heat leakage (along their drive shafts, for example) that causes boiling of the cryogen and the same effects as cavitation.
The invention is a pump having a pump housing defining a chamber, and a free piston located within the chamber. The free piston defines one (or more) pumping chamber(s). Each pumping chamber receives fluid from one (or more) inlet port(s) and delivers liquid to one (or more) outlet port(s). The pump has an electromagnetic drive system for moving the piston within the chamber.
In one embodiment, the chamber and the piston are cylindrical. However, the chamber and the piston can have other shapes for their cross-sections, including the rectangular shape.
In one embodiment, the electromagnetic drive system is comprised of a power source, one (or more) drive element(s) secured to the pump housing, and the free piston (which acts as the driven element of the electromagnetic drive system). In another embodiment, the free piston is secured to a driven element and the assembly defines one (or more) pumping chamber(s) and one (or more) driven-element chamber(s). Fluid from the supply enters and leaves each driven-element chamber via one (or more) driven-element chamber port(s). The electromagnetic drive system in this embodiment is comprised of a power source, one (or more) drive element(s) secured to the pump housing, and the driven element secured to the free piston. Each drive element can be a solenoid coil, and the driven element can be a solenoid plunger.
In one embodiment, an inlet porting system allows liquid to enter each pumping chamber when the pressure in the pumping chamber is low. In another embodiment, an inlet check valve admits liquid into the pumping chamber when the pressure in the pumping chamber is low. In a preferred embodiment, each inlet check valve contains a poppet with a small mass and (therefore) inertia.
In one embodiment of the invention, an outlet porting system allows liquid to exit each pumping chamber when the pressure in the pumping chamber is high. In a preferred embodiment, the outlet porting system is contained in an injection needle. The injection needle is a small-diameter tube with most of its bore filled, except passages at each end between holes drilled radially through the wall near each end and near the place where the injection needle passes into the free piston, to which the injection needle is secured. The injection needle passes through a small-diameter passage that separates each pumping chamber from each outlet port. In another embodiment, an outlet check valve lets the liquid exit each pumping chamber when the pressure in the pumping chamber is high. In a preferred embodiment, the outlet check valve contains a poppet with a small mass and (therefore) inertia.
In one embodiment, the power source alternately supplies a solenoid coil with: (1) a positive voltage to induce a positive current through the solenoid coil; then (2) a negative voltage to reduce the current in the solenoid coil; and then (3) zero voltage when the current in the solenoid coil reaches a zero value. In a preferred embodiment, the power supply contains a capacitor that stores electrical energy drawn from the solenoid coil and returns the stored energy to the solenoid coil at a later time.
The pump can use three different types of bearings to separate the free piston from the pump housing: (1) rubbing bearings; (2) hydrostatic bearings; and (3) hydrodynamic bearings. These types of bearings can be used alone, or combinations of the three can be used together. In one embodiment, rubbing bearings are used, and the rubbing bearings are comprised of o-rings secured to the pump housing, which rub against the free piston. In another embodiment, the o-rings are secured to the free piston, and the o-rings rub against the chamber walls of the pump housing. In a preferred embodiment, the o-rings are made of polytetrafluoroethylene (PTFE). However, in other embodiments, the o-rings are made of different materials. For example, the o-rings can be made of materials that are soft (brass, for example) relative to the materials from which the free piston and pump housing are fabricated (stainless steel, for example).
In one embodiment, hydrostatic bearings separate the free piston from the pump housing. A key component of a hydrostatic bearing is a restrictor. In preferred embodiments, the hydrostatic bearings use screws to perform the restrictor function of the bearings. Screws can be used to form three different types of restrictors: (1) laser-drilled restrictors; (2) mechanically drilled restrictors; and (3) thread restrictors. For laser-drilled restrictors, a hole is laser-drilled down the axis of each screw. For mechanically drilled restrictors, a hole is drilled down the axis of each screw with a drill bit. For thread restrictors, the gap between the threads of each screw and the threaded hole in the pump housing functions as the restrictor in the bearings.
In one embodiment, hydrodynamic bearings separate the free piston from the pump housing. In a preferred embodiment, the hydrodynamic bearings are step-slider bearings. The steps of the step-slider bearings can be on the free piston or the chamber walls of the pump housing.
The pump contains a means for limiting leakage of liquid from the pumping chamber when the pressure in the pumping chamber is high. In one embodiment, clearance seals are used. In another embodiment, rubbing seals are used.